This has been called the reptile age because there were such numbers of animals like crocodiles, lizards, and tortoises (which are all reptiles), and some of them were of immense size. For instance, there was a huge creature something like a frog, but as large as a Shetland pony, called the _Labyrinthodon_, with a great many curious teeth, and this animal has left footprints in the New Red Sandstone which have been dried and buried, we can’t tell how long, and there are the cracks made by the sun drying the place he walked over when that was soft earth. There is a drawing of some of these footsteps in the picture on the next page, and there are also the footprints of a large bird, and you can see where he walked over the soft earth and made a long line of footmarks; and if you look at the footprints of birds on the snow or mud now you will notice marks just like these. Then there is another picture of a single footprint of a large bird, and all those round dots are where rain-drops fell and left their marks in the soft earth. [Illustration: IV. (1) _Footprints of Labyrinthodon._ (3) _Footprints of Birds_, (2) _with marks of Rain-drops_.] I dare say you will wonder how it is that these footprints have not disappeared. Well, when the animals and birds that made them had gone the marks became filled with dry sand, no doubt blown in by the wind, and then the mud dried hard, and at last it became covered with other earths and sank slowly down, just as the coal forests had done before, and remained there until we dug it up with these tracks of the birds and animals that lived then. Some of these birds must have been larger than any living now, because their footmarks are so long. None of their bones have been found yet, I believe, but plenty of the teeth and some bones of the labyrinthodon have. The real footmarks, of course, are very large, though they are small in the picture.[9] In the great beds of Lias there are many other strange animals, and among them are two great fish-lizards called the _Ichthyosaurus_ and _Plesiosaurus_. Both of these lived in the water and perhaps came on land sometimes, and it is certain that they must have been very ferocious creatures, from their great size and sharp teeth. The plesiosaurus would be able to raise his long neck above the water and snap at some of those curious birds rather like bats which lived at the time, and of which I shall have something to say presently. Some of these fish-lizards were as large as whales, and their bodies have been so beautifully preserved in the limestone rocks that we can actually sometimes find in their stomachs the food they lived on. [Illustration: V. _Ichthyosaurus._ _Plesiosaurus._ FISH-REPTILES.]